Question

Complete sentences are generally advised, but some questions may be answered with bullets or lists--depending on the context.

1. Explain the factors responsible for the "dead zone" where the Mississippi R. meets the Gulf of Mexico.
2. Describe the “whole-lake” experiment conducted at the Experimental Lakes Area in Canada which demonstrated that phosphorus (P) was the nutrient most limiting algal productivity in lakes.
3. Explain why toxic compounds usually have the greatest effect on top-level carnivores.
4. Describe the term ‘trophic efficiency’ (energetically) and provide an example of it in a food chain.

Answer 1

1) Nutrient enrichment from the Mississippi River causes the dead zone, particularly nitrogen and phosphorous. Watersheds within the Mississippi River System swamp all of the United States from Montana to Pennsylvania and run down the Mississippi River southwards. The majority of nitrogen production comes from large Mississippi River Valley agricultural states including Minnesota, Iowa, Illinois, Wisconsin, Missouri, Tennessee, Arkansas, Mississippi , and Louisiana. Nitrogen and phosphorous enter the river via upstream fertilizer runoff, soil erosion, animal wastes, and sewage.Such nutrients are not major factors in algae production in a natural environment since they are absorbed by plants in the soil. With anthropogenically enhanced intakes of nitrogen and phosphorus, algae production is no longer restricted, however. Therefore, algal blooms form, the food chain is changed, and dissolved oxygen is reduced in the environment. The scale of the dead zone fluctuates seasonally, as the farming activities intensify it. It often gets influenced by environmental disasters like earthquakes and hurricanes.

3) Toxins are ingested by organisms and accumulate in the fatty tissues These toxins become more concentrated in successive trophic levels of a food wed- biological magnification.Magnification occurs because the biomass at any given trophic level is produced from a much larger biomass ingested from the level below.Thus, top-level carnivores tend to be the organisms most severely affected by toxic compounds

4) Trophic efficiency The ratio of production at one trophic level to production at the next lower trophic level. is calculated by the percentage of energy that consumers in one trophic level gain and convert into biomass from the total stored energy of the previous trophic level.For example, when an herbivore (such as a grass-eating insect) consumes 100 kcal (kilo calories) of grass, it cannot convert 100% of those kcal into insect biomass. Instead, some of the energy is used in the insects' searching to find suitable food, and in eating and digesting the food.